Controlled cellular orientation and outgrowth is commonly seen in natural tissues and is closely related to tissue function. For example, in arteries, smooth muscle cells (SMCs) and collagen fibrils are circumferentially oriented at the medial layer to provide mechanical support against circulatory blood pressure (see, e.g., Nerem, R. M. and D. Seliktar, Annual Review of Biomedical Engineering, 2001. 3: p. 225-243; Vaz, C. M., et al., Acta Biomaterialia, 2005. 1(5): p. 575-582). In the adult myocardium, cardiomyocytes elongate and orient in parallel to form a syncytium, which enables propagation of electrical signals (see, e.g., Eschenhagen, T. and W. H. Zimmermann, Circulation Research, 2005. 97(12): p. 1220-1231). Enamel, which provides superb mechanical properties to teeth, is formed by highly aligned ameloblasts producing hydroxyapatite crystals in an ordered manner (see, e.g., Nishikawa, S., Anatomical Record, 1992. 232(4): p. 466-478). The successful formation of neural circuits in vitro and nerve regeneration in vivo also depends on guiding neuronal growth cones along specific pathways to help them find correct targets (see, e.g., Dickson, B. J., Science, 2002. 298(5600): p. 1959-1964). Reproducing these “in-vivo-like” orientation and organization of the cells in an engineering system therefore is a very intriguing and challenging subject.
The established “contact guidance” theory illustrates that in many cases cell or cell process has bi-directional response to anisotropic chemical, structural and/or mechanical property of the substratum (see, e.g., Bellairs R, C. A., Dunn G, Cell behavior. Cambridge: Cambridge University Press, 1982: p. 247-280; Tranquillo, R. T., Biochem Soc Symp, 1999. 65: p. 27-42). Based on this theory, researchers have successfully used techniques such as electro-spinning (see, e.g., Xu, C. Y., et al., Biomaterials, 2004. 25(5): p. 877-86; Yang, F., et al., Biomaterials, 2005. 26(15): p. 2603-2610), laser nanotopography (see, e.g., Zhu, B., et al., Biomaterials, 2004. 25(18): p. 4215-23), micro-contact printing (see, e.g., Schmalenberg, K. E. and K. E. Uhrich, Biomaterials, 2005. 26(12): p. 1423-30; Wang, D. Y., et al., J Biomed Mater Res B Appl Biomater, 2007. 80(2): p. 447-53), microfabrication and mircromachining (see, e.g., Lee, P., et al., Biomed Microdevices, 2006. 8(1): p. 35-41; Mata, A., et al., Biomedical Microdevices, 2002. 4(4): p. 267-275; Charest, J. L., A. J. Garcia, and W. P. King, Biomaterials, 2007. 28(13): p. 2202-10) to create patterned substrates and demonstrated the capability to orient cells in monolayer tissue culture. However, tissue engineering (TE) scaffolds are structurally distinct in that they are three-dimensional (3-D). It is thus more attractive to realize alignment of cells in a 3-D environment. Some earlier researchers achieved this by using dynamic culture conditions (see, e.g., Kanda, K. and T. Matsuda, Cell Transplant, 1994. 3(6): p. 481-92) and gradient chemotropic guidance (see, e.g., Tessierlavigne, M., et al., Nature, 1988. 336(6201): p. 775-778). More recently, researchers developed photo labile hydrogels, and used light to guide cell growth (see, e.g., Luo, Y. and M. S. Shoichet, Nature Materials, 2004. 3(4): p. 249-253). However, a more general and simpler method is still preferred in order to fabricate sophisticated engineering devices.
Improved methods for generating and using aligned nanofiber bundle assemblies are needed.